The following bug has been discovered when using AssistiveTouch:
If the AssistiveTouch panel is the last thing pressed on the screen (such as to return it to it's minimized size), the screen will *not* AutoLock if left alone as it should. It will stay on indefinitely until the battery is depleted.
The work-around is to make any screen touch after touching the AssistiveTouch panel (like a screen swipe) to allow the unit to AutoLock correctly.
The Apple forums link to the discussion is (I am SergZak in the Apple forums):